Not only is a shortage of trained officers and issue, but most police forces are unable to take the blood samples needed as evidence for any prosecution.

One option would be to hire a nurse or technician.

Canada is looking at providing officers with an oral fluid device which can help detect the presence of drugs.

Even without the legalisation of cannabis, police forces around the world have been wrestling with the problem of how to act against drug-impaired drivers.

Canada preparing for marijuana legalisationCredit:
Steven Senne/AP

Similar challenges are being faced in the US where recreational marijuana use is now legal in nine states.

In Britain, where cannabis is still outlawed, legislation setting prescribed limits of a number of recreational drugs were introduced as recently as 2015, when roadside tests were brought in to screen for cannabis and cocaine.

According to figures released in March from 40 police forces across the UK, 8,336 drivers tested positive for cannabis and 3,064 for cocaine between March 2015 and January 2018.